Breaking the Boundaries of the Goldschmidt Tolerance Factor with Ethylammonium Lead Iodide Perovskite Nanocrystals

We report the synthesis of ethylammonium lead iodide (EAPbI₃) colloidal nanocrystals as another member of the lead halide perovskites family. The insertion of an unusually large A-cation (274 pm in diameter) in the perovskite structure, hitherto considered unlikely due to the unfavorable Goldschmidt tolerance factor, results in a significantly larger lattice parameter compared to the Cs-, methylammonium- and formamidinium-based lead halide perovskite homologues. As a consequence, EAPbI₃ nanocrystals are highly unstable, evolving to a nonperovskite δ-EAPbI₃ polymorph within 1 day. Also, EAPbI₃ nanocrystals are very sensitive to electron irradiation and quickly degrade to PbI₂ upon exposure to the electron beam, following a mechanism similar to that of other hybrid lead iodide perovskites (although degradation can be reduced by partially replacing the EA⁺ ions with Cs⁺ ions). Interestingly, in some cases during this degradation the formation of an epitaxial interface between (EA_xCs_1-x)PbI₃ and PbI₂ is observed. The photoluminescence emission of the EAPbI₃ perovskite nanocrystals, albeit being characterized by a low quantum yield (∼1%), can be tuned in the 664-690 nm range by regulating their size during the synthesis. The emission efficiency can be improved upon partial alloying at the A site with Cs⁺ or formamidinium cations. Furthermore, the morphology of the EAPbI₃ nanocrystals can be chosen to be either nanocube or nanoplatelet, depending on the synthesis conditions.